A) Field of the Invention
The present invention relates to an optical transmission device, and more particularly to an optical transmission device for optically coupling an optical waveguide formed on a substrate to another optical device.
B) Description of the Related Art
Optical communication is increasing its speed and capacity because of a broadening transmission band and development of wavelength division multiplexing. In order to configure a hardware infrastructure of an optical fiber network in a trunk communications network, optical switches are required for switching optical signals toward destinations.
An example of an optical switch is shown in FIG. 9A. The optical switch includes a plurality of optical splitters 100, an optical switch module 101, a plurality of optical multiplexers 110, and a plurality of optical amplifiers 111. An optical fiber 120 is connected to each optical splitter 100. A wavelength division multiplexed optical signal is supplied from the optical fiber 120 to the optical splitter 100. The optical splitter 100 splits the wavelength division multiplexed optical signal into a plurality of optical signals. Split optical signals are input to the optical switch module 101 at the succeeding stage.
The optical switch module 101 has a three-stage structure. Each stage is constituted of a plurality of optical switch substrates. At the first stage, the optical switch substrate is provided for each optical splitter 100 to switch optical signals from optical waveguides of each optical splitter 100. The optical switch substrate at the second stage switches optical signals from a plurality of optical switch substrates at the first stage. The optical switch substrate at the third stage switches optical signals from a plurality of optical switch substrates at the second stage.
The optical multiplexer 110 is provided for each optical switch substrate at the third stage to multiplex the optical signal output from each optical switch substrate at the third stage. The multiplexed optical signal is amplified by the optical amplifier 111. An optical connector 115 is provided for connection between the optical splitter 100 and optical switch substrate at the first stage of the optical switch module 101, between the optical switch substrate at each stage of the optical switch module 101 and optical switch substrate at the succeeding stage, and between the optical switch substrate at the third stage and optical multiplexer 110.
FIG. 9B is a plan view of the optical switch substrate of the optical switch module 101 shown in FIG. 9A. An XY rectangular coordinate system is defined on the surface of a rectangular substrate 125, the X- and Y-axes being parallel to the sides of the rectangle. A plurality of input side optical waveguides 130 are disposed along one side parallel to the Y-axis to transmit light along the X-axis direction. A collimator lens 131 and a beam deflection element 132 are disposed on the surface of the substrate 125 in correspondence with each input side optical waveguide 130.
A beam deflection element 134 on the output side is disposed in correspondence with each beam deflection element 132, with a slab optical waveguide 133 being interposed therebetween. A condenser lens 135 and output side optical waveguide 136 are disposed in correspondence with each beam deflection element 134.
The input side beam deflection element 132 changes the propagation direction of a light beam in the substrate plane. The light beam with a changed propagation direction propagates through the slab optical waveguide 133 and becomes incident upon the output side beam deflection element 134. The beam deflection element 134 changes the propagation direction of the light beam to make it incident upon the corresponding condenser lens 135. The condenser lens 135 converges the light beam at the input end of the corresponding output side optical waveguide 136.
By deflecting a light beam to a desired direction by the input side beam deflection element 132, the optical signal input to the input side optical waveguide 130 can reach a desired output side optical waveguide 136. An optical signal can be switched by controlling the deflection direction at each time slot of the optical signal.
A method of connecting the output side optical waveguide 136 shown in FIG. 9B to the input side optical waveguide of, for example, the optical multiplexer 110 shown in FIG. 9A, is disclosed in JP-A-2000-304966 and JP-A-5-40214.
According to the invention disclosed in JP-A-2000-304966, a lens is disposed in correspondence with each output side optical waveguide between the output side and input side optical waveguides. Each lens converges light output and diverged from a corresponding output side optical waveguide at the input end of the corresponding input side optical waveguide. Since the output ends of the output side optical waveguides are disposed in line, the lenses are made of a micro lens array.
According to the invention disclosed in JP-A-5-40214, a collimator lens and a condenser lens are disposed in correspondence with each output side optical waveguide between the output side and input side optical waveguides. Light output and diverged from each output side optical waveguide is changed to a parallel light flux by a corresponding collimator lens, and this parallel light flux is converged at the input end of the input side optical waveguide by the condenser lens. These collimator lenses and condenser lenses are also made of micro lens arrays. Since the light beam between the collimator lens and condenser lens is a parallel light flux, a position alignment precision of a space between the collimator lenses and condenser lenses can be relaxed. Since the lenses have a sealing structure, the inside of the optical system can be protected. The influence of attached dusts can be mitigated.
JP-A-5-264874 discloses an optical system of converging light radiated from a light source and makes the light incident upon the input end of an optical fiber. By utilizing a thermal expansion of components for mounting optical elements, a change in the focal length of a lens to be caused by a temperature change can be compensated.
A position displacement between an optical waveguide and a lens to be caused by a temperature change is required to be suppressed in order to maintain high a coupling efficiency between the output side and input side optical waveguides. A position displacement (along a direction parallel to the propagation direction of a light beam) to be caused by a change in the focal length of a lens to be caused by a temperature change can be compensated by the method of utilizing the thermal expansion of mount components disclosed in JP-A-5-264874. If the optical waveguide is of a single mode, the position precision of 1 μm or smaller is necessary with respect to two directions perpendicular to the propagation direction of a light beam.
If lenses are made of a micro lens array, a distance between lenses changes because of thermal expansion of lens material. If the positions of a particular optical waveguide and a particular lens are set at a high precision, the positions of other optical waveguides and lenses are displaced.